Glutamate (Glu), acting at AMPA/kainate (non-NMDA) receptors, appears to be the major fast excitatory neurotransmitter in the vertebrate central nervous system (CNS), participating in process ranging from learning and memory to excitotoxic neurodegeneration. The excitatory and toxic actions of Glu result from the flow of cations through Glu-gated ion channels. This makes it important to understand factors that regulate neuronal Glu responses. Recent evidence suggests that Glu can modulate its own actions at AMPA receptors by at least two mechanisms - presynaptic inhibition of transmitter release and rapid postsynaptic receptor desensitization. Both of these regulatory actions occur at Glu concentrations similar to those found in the CNS extracellular fluid, suggesting that Glu exerts a tonic influence over excitatory neurotransmission. Additionally, studies from our laboratory suggest that AMPA receptors have a regulatory site at which the novel benzodiazepine, GYKI 52466, and the benzothiadiazide, cyclothiazide, act to alter desensitization. In this proposal, experiments are described that will examine the actions and interactions of GYKI 52466 and benzothiadiazides at AMPA receptors on cultured postnatal rat hippocampal neurons using whole-cell and outside-out patch clamp recordings. Using the benzothiadiazides as tools we will also examine the role of presynaptic inhibition and postsynaptic desensitization in the modulation of fast monosynaptic excitatory synaptic responses by micromolar Glu concentrations. Other experiments will examine the contribution of pre and postsynaptic Glu effects to short-term forms of synaptic plasticity. These studies will examine paired-pulse and stimulus train-train-induced synaptic changes in microisland hippocampal cultures where monosynaptic excitatory synapses can be studied in isolation. Finally, we will examine the contribution of pre-and postsynaptic changes to longer-term forms of synaptic plasticity in rat hippocampal slices. These experiments will address whether changes in AMPA receptor desensitization or Glu-mediated presynaptic inhibition contribute to synaptic modifications that accompany long-term potentiation (LTP) and long-term depression (LTD) in the CA1 region. These studies have the potential to shed light on factors regulating fast Glu-mediated synaptic transmission and possibly on the role of the Glu system in certain forms of learning and memory.